The present invention relates to an electric power steering system for a motor vehicle, and more particularly to a system for controlling a motor in accordance with the turning of a steering wheel.
The electric power steering system has a torsion torque sensor for measuring torsion torque in a steering shaft. The system is adapted to produce an assist signal to control the rotational direction and torque of a motor in dependency on a torsion torque signal from the sensor (Japanese Patent Application Publication No. 45-41246). The response of the steering system and the returnability of the steering wheel may deteriorate because of the inertia of the motor, the friction of a reduction gear train provided in the system, and other factors.
A control system has been proposed which may prevent the above-described disadvantages (U.S. Pat. No. 4,664,211). The system is provided for producing an auxiliary assist signal dependent on the differential of the torsion torque signal from the torsion torque sensor, a return torque signal depending on an output signal of a steering angle sensor, for returning the steering wheel to a neutral position, and an assist signal depending on the torsion torque signal which is corrected in accordance with vehicle speed. These signal are added for controlling the motor.
In such a system, when the steering wheel at an angular position is rotated in the torsion torque decreasing direction for example the steering wheel is rotated (to the neutral position) by the driver's hand, an assist torque including a return direction torque in the torsion torque decreasing direction is generated, thereby reducing the steering effort. However, if the auxiliary assist signal is set to a value suitable for the operation in the torsion torque increasing direction (FIG. 3), the assist torque including return direction torque in the torsion torque decreasing direction becomes extremely large for returning the steering wheel. As a result, the steering wheel is returned by the driver at a very small force at higher speed than the driver estimates.
Such a high speed return of the steering wheel diminishes steerability of the vehicle and provides an unpleasant feeling to the driver.